Photopolymerizable compositions prepared from ethylenically unsaturated monomeric compounds, photoinitiators, organic polymeric binders as well as other additives are known to be useful for the preparation of dry film resists and for other uses. The components that are used in the preparation of the photopolymerizable compositions are generally mixed in a suitable solvent or solvents for the polymeric binder, and the composition is formed into a dry film by coating and removing the solvent, e.g., by extrusion coating, or other means to form dry films known to those skilled in the art.
Photopolymerizable compositions, particularly in dry layer form, are becoming increasingly popular for printing plates and photoresist films. Dry film photoresists, for example, have many advantages over liquid photoresists in the manufacture of printing circuits. In fact, dry film photoresists are becoming so popular that manufacturing capability of the plant may quickly be reached. One problem that occurs in the manufacture of photoresist films is the ability of such films to be dried quickly and efficiently. Drying of photopolymerizable coatings in a hot air dryer is influenced by many variables, e.g., thickness and solids concentration of the coatings, characteristics of the solvents, solvent interaction with the composition to be coated, temperature of the film, temperature of the air, air velocity, direction and turbulence, relative humidity, etc. Generally, drying may be considered to take place in two stages. In the first stage, the rate of drying is about the same as the rate for removal of pure solvent, the limiting factor is the rate at which solvent molecules can be removed from the surface (or from the air space above the surface) of the coating. The supply of heat energy to the coating and the air velocity are significant factors in this first drying stage. In the second stage, as the coating becomes more concentrated, drying slows down and is limited by the rate of diffusion of solvent molecules from the interior of the coating to the air surface of the coating. In this second drying stage not only are the temperature and air velocity important but some internal factors, e.g., the interaction between the solvent and solute, etc., are important as well.
In view of the many problems associated with the drying of photopolymerizable coatings, it is known that the temperature used to dry such coatings, should be less than the temperature at which bubbles form in the coating. The formation of bubbles is undesirable because once formed the bubbles may not collapse or if they do break the coating may be too viscous and uneven spots are formed in the layer. As a result dryers are usually arranged in zones, the temperature in each zone being held at some safe margin below the temperature that will cause bubbles to appear in the coating.
An additional limitation on the drying rate is the tendency of polymer solutions to "skin over". Too-rapid drying, especially in initial stages, results in formation of a substantially dry skin on the surface of the coating, but solvent-rich coating is stil present below the skin which once dried does not readily redissolve in the underlying solvent. The skin thus acts as a barrier to solvent evaporation. This problem may be alleviated by slower drying, e.g., lower temperature, air velocity, adding a higher boiling solvent, etc. Slower drying, however, results in a slower production of photopolymerizable films and elements.
It has been found surprisingly that by replacing a portion of the dissolved polymeric binder of a photopolymerizable coating composition with crosslinked polymeric beads which are insoluble in the coating solvent dry photopolymerizable films and elements can be prepared more efficiently. The photopolymerizable layers of such films and elements dry more quickly with less risk of bubble formation. Production can thereby be increased even though there is no increase in actual size of the manufacturing facility. The photopolymerizable layer has been found to retain desirable properties in spite of the presence of the crosslinked polymeric beads since they can be designed to closely match the refractive index of the soluble polymeric binder thereby minimizing light scattering often associated with inorganic insoluble fillers. For example, the photographic speed, adhesion to copper, clarity or resolution, and flexibility of the photopolymerizable layer are maintained.